In many Unix operating systems the network devices have appearances in both the filesystem namespace (e.g. the /dev directory) and the network interface namespace. This is not so in Linux, where no character devices are used.
In the majority of cases, the network interface is automatically created by the device driver while it is initializing after it has located your hardware. For example, the ethernet device driver creates eth[0..n] interfaces sequentially as drivers locate your ethernet hardware. The first ethernet card found becomes eth0, the second eth1, etc.
In some cases though, notably slip and ppp, the network devices are created through the action of some user program. The same sequential device numbering applies, but the devices are not created automatically at boot time. The reason for this is that, unlike ethernet devices, the number of active slip or ppp devices may vary during the uptime of the machine. These cases will be covered in more detail in later sections.